1. Field of the Invention
The present invention relates, in general, to garage door operators and, more specifically, to motor controls for garage door operators.
2. Description of the Art
Garage door operators or openers responsive to manually controlled switches directly wired to the controller or switches coupled by radio signals to the controller are well known. The controller normally opens the door from a closed or partially closed position when the switch is actuated and closes the door if it is open when the switch is actuated.
It is also known to provide garage door operators or openers with upper and lower position limits, and with sensitivity or torque limits. Generally, manual adjustment or selection is provided to allow the user or the installer of the door operator to set position limits which coincide with the fully opened and fully closed positions of the door, and to set sensitivity limits which permit sufficient torque throughout the complete range of door movement in both opening and closing directions, but with not enough torque to damage the door. The sensitivity setting and the position limits are also used in obstruction detection to stop the door in time to prevent substantial damage to the obstruction. The forms of obstruction detection are many and include mechanical sensors in the lower portion of the door, motor load monitoring devices, and run time circuits.
It is also known to employ microprocessors in the controller to determine limits, activation code signals, obstruction detection, door position, etc. FIG. 1 shows a prior art use of a thermal overload in a motor control circuit of a garage door operator. The thermal overload switch is connected to the neutral power line and the motor windings. The thermal overload switch is set for a predetermined overload temperature, such as 130.degree. C., for example. If an overload occurs due to a stalled door, for example, the temperature of the motor windings increases causing the thermal overload switch to open and cut current to the motor to thereby deactivate the motor and the control circuit.
However, when the thermal overload switch is actuated, the disconnection of electrical power stops door movement in the last position of the door, such as fully closed, fully opened or varying degrees of partially opened or closed. Depending on the overload temperature, it may take from 1 to 15 minutes for the thermal overload switch to automatically reset to a closed state reapplying power to the motor windings.
Dual thermal overload switches have been employed in motor circuits, specifically in separate power lines, but not with garage door operator motors which still enable partial door operation after activation of any one thermal overload switch.
Thus, it is desirable to provide a garage door operator motor control circuit having dual thermal overload means which enables partial door and motor operation in the event of a thermal overload event. It would also be desirable to provide a garage door operator with a secondary thermal overload means for garage door opening movement to a fully opened position prior to activation of the main or primary thermal overload means.